1. Field of the Invention
The present invention relates to a method of audio signal-processing for a loudspeaker located close to an ear, and particularly, though not exclusively, to headphone xe2x80x9cvirtualisationxe2x80x9d technology, in which an audio signal is processed such that, when it is auditioned using headphones the source of the sound appears to originate outside the head of the listener.
2. Background
Conventional stereo audio creates sound-images which appearxe2x80x94for the most partxe2x80x94to originate inside the head of the listener, because of the absence of three-dimensional sound-cues. At the present time, there are no adequate and efficient methods for creating a truly effective xe2x80x9cout-of-the-headxe2x80x9d external sound image, although this has been a long sought-after goal of many audio researchers.
By measuring so-called xe2x80x9cHead-Related Transfer Functionsxe2x80x9d (HRTFs) from a sound-source at specified locations in space, the spatially dependent acoustic processes which act on the incoming sound-waves, caused by the head and outer ear, can be synthesised electronically. This processing, when applied to an audio recording and auditioned on headphones, creates the auditory illusion that the listener hears the recording from a sound-source at that point in space corresponding to the spatial position associated with the HRTF. However, this method is anechoic (no sound-wave reflections are present), and emulates listening to the sounds in an anechoic chamber. The consequent effect is that, although the direction of the sound-source can be emulated reasonably well, its distance is impossible to judge. The sound-source appears to be situated very close to the head.
If an element of artificial reverberation is added to the above processing, then the illusion of providing an external sound-image can be improved a little, but the effects are still not convincing. This is well known for stereo signals, and has been described in our co-pending patent application GB 0009287.4 for monophonic signals.
However, it is known that more adequate xe2x80x9cexternalisationxe2x80x9d effects can sometimes be demonstrated by means of artificial-head recordings, but the recording method does not lend itself to synthesis. Similarly, various so-called xe2x80x9cauralisationxe2x80x9d signal-processing technologies have been known to create adequate externalisation effects by replicating the impulse response of the entire reverberant properties of a chosen room (typically lasting 4 or more seconds). However, this is achieved at the expense of massive signal-processing effort which is prohibitively impractical for incorporating into, say, portable stereo players, even by present-day standards.
It is an object of the present invention to provide an effective method for creating an external sound-image for headphone listeners, which (a) uses minimal and practicable signal-processing, and (b) which is xe2x80x9cneutralxe2x80x9d, in the sense that it does not necessarily possess specific room characteristics, such that it could be used in conjunction with many different reverberation types, if required.
According to a first aspect of the present invention, there is provided a method as specified in claims 1-7. A second aspect of the invention provides apparatus as specified in claims 9-13, whilst a third aspect of the invention provides an audio signal as specified in claim 8.